Inverter providing a variable output voltage

ABSTRACT

An inverter circuit for supplying an AC output from a DC input. The circuit comprises a bridge circuit including four controlled diodes, one in each leg of the bridge. The diodes are fired by a control circuit in such a manner that an AC output is supplied to a load connected across a diagonal of the bridge.

United States Patent Us] 3,694,732 Nollace 1 Sept. 26, 1972 [$4]INVERTER PROVIDING A VARIABLE [56] References Cited OUTPUT VOLTAGEUNITED STATES PATENTS [721 L en 9?? 3,378,751 4/1968 Walker ..321/13Assignee; commie Gengrfle DElggtmnique 3,461,373 v 8/1969 Mokrytzki..32l/9 A lndustrielie bepaute, Paris, France a Primary Examiner-WilliamM. Shoop, Jr. Fllfidi Zz, Attorney-Sughrue, Rothwell, Mion, Zinn &Macpeak 21 l. l l 51 v ABSTRACT 0"! Hum An inverter circuit forsupplying an AC output from a [30] F pmuon y DC input. The circuitcomprises a bridge circuit inp |969 France-i ....6932445 eluding fourcontrolled diodes, one in each leg of the bridge; The diodes are firedby a control circuit in [52] US. Cl. ..32l/45 R such a manner that an ACoutput is supplied to a load [5 1] Int. Cl..- .JIOZIn 7/48 connectedacross a diagonal of the bridge.

58 FieldoISeu-eh ..32l 9A, 18, 45 I l 6 Claims, 6 Drawing FiguresPKTENTED I972 2O l l 21 23 l l t 2s| I F I I In I 22 24| L' OFF OFF

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OFF ON E IN VENTOR PIERRE NOLLACE BY j flxe (KM, 24 -1 A. 9 AmJPq KATTORNEYS INVERTER PROVIDING A VARIABLE OUTPUT VOLTAGE The presentinvention concerns an inverter comprising two pairs of controlledrectifiers connected in a bridge circuit with an input across a firstbridge diagonal for connection to a direct current supply and an .outputacross the second bridge diagonal for connection to an alternatingcurrent load, the output voltage of the inverter being variable.

The controlled rectifiers of one pair have their anodes connected to apositive terminal of the input and the controlled rectifiers of theother pair have their cathodes connected to a negative terminal. Thusthe inverter consists of a series-connected combination of rectifiersconnected on each side of the load.

Systems for varying the output voltage of an inverter have been proposedin which the firing times of the controlled rectifiers of the inverterare varied. In these systems, each controlled rectifier is renderedconductive during a'half period of the output and the regulation'of theoutput voltage is obtained by varying the phasing of the control signalsapplied to the controlled rectifiers of one series-connected combinationin relation to the control signals applied to the other combination. 1

In such an arrangement; the reference potential of the alternatingcircuit is sometimes that of the positive supply terminal and sometimesthat of the negative. Thus it is not possible to obtain a cold pointcommon to the power circuit and the control circuit, that is, a pointwhose potential may serve as a reference for the two circuits. It isvery advantageous, however, in this type of equipment, to provide such areference point allowing, for example, the two parts of the equipment tobe connected to a common return, such as a ground connection.

In accordance with this invention, an inverter comprises two pairs ofcontrolled rectifiers connected in a bridge circuit with an input acrossa first bridge diagonal for connection to a directcurrent supply and anoutput across the second bridge diagonal for connection to analternating current load. The controlled rectifiers of one pair havetheir anodes connected to a positive terminal of the input and thecontrolled rectifiers of the other pair have their cathodes connected toa negative terminal of the input. The output voltage of an inverter isvaried by controlling the firing times of the controlled rectifiers, therectifiers of a first pair being maintained continuously in conductionexcept during periods at which power is to be supplied to the inverteroutput, at which point one of the conducting rectifiers is extinguishedand the rectifier of the second pair in series with it is fired.

In this way, the reference potential of the alternating voltage providedis equal to the potential of the direct current supply since the pair ofcontrolled rectifiers are maintained continuously in conduction exceptduring those periods at which power is to be supplied to the inverteroutput. v

The inverter preferably includes a control circuit for the controlledrectifiers arranged to provide a first train of pulses at twice theinverter output frequency and a second train of pulses delayed withrespect to the first train by an amount dependent on the alternatingoutput voltage of the inverter. The pulses of the first train extinguishone of the continuously conducting rectifiers of the first pair and firethe rectifier of the second pair in series with it. The pulses of thesecond train reverse this action by refiring the extinguished rectifierand extinguishing the fired rectifier.

Alternatively, the inverter may include a control circuit for thecontrolled rectifiers arranged to provide first and second train ofpulses with periods equal to that of the alternating inverter outputvoltage. The leading edge of a pulse will extinguish one of thecontinuously conducting rectifiers of the first pair and fire therectifier of the second pair in series with it. The end of the pulsewill reverse this action by refiring the extinguished rectifier andextinguishing the fired rectifier.

The invention will now be described in more detail, by way of examplesonly and with reference to the accompanying diagrammatic drawings, inwhich:

FIG. 1 is a schematic diagram of the preferred embodiment of aninverter;

FIG. 2 is a waveform diagram referred to in the explanation ,of theoperation of the circuit of FIG. 1;

FIG. 3 shows waveforms at various points in the circuit of FIG. 1 on lowload;

FIG. 4 shows waveforms at various points in the circuit of FIG. 1 onfull load;

FIG. 5 shows a first form of control circuitry for the inverter; and

FIG. 6 shows a second form of control circuitry.

Referring to FIG. 1, the inverter is shown in a simplified form, andonly the four controlled rectifiers 11, 12, 21 and 22 are shown. Thefour controlled rectifiers are connected in a bridge circuit with aninput across a first bridge diagonal for connection to a direct currentsupply and an output across the second bridge diagonal for connection toan alternating current load. One pair of controlled rectifiers l1 and 21have their anodes connected to a positive terminal 1 of the input andthe other pair of controlled rectifiers 12 and 22 have their cathodesconnected to a negative terminal 2 of the input. The direct inputvoltage is indicated at E The alternating current load is shown at 3connected between points 15 and 25. Also connected to 15 and 25 are thecathodes of rectifiers 11 and 21 and anode of rectifiers l2 and 22.Diodes 13, 14, 21 and 22 are connected in anode-cathode parallelrelationship with the rectifiers 11, 12, 21 and 22, respectively, toprovide return paths.

In FIG. 1, other controlled rectifiers or equivalent switching circuitsconnected to the control gates of the controlled rectifiers 11, 12, 21and 22 have not been shown, for the sake of simplicity in the diagram.Each pair of controlled rectifiers l l, 12, and 21, 22 provides aseries-connected combination, each connected to one side of the load 3.Filter circuitry (not shown) is also connected between the points 15 and25.

In FIG. 2, the waveforms 2a to 2d show how the states of the respectivecontrolled rectifiers ll, 12, 21 and 22 vary with time, and waveform 2eshows the voltage obtained between points 15 and 25. In waveforms 2a to2d, the curve coincides with the axis of time when the correspondingcontrolled rectifier is conducting. When a rectifier is cut-off orextinguished, the corresponding curve consists of a straight lineparallel to the time axis and spaced therefrom. The de-ionizationperiods have not been shown in this diagram.

It is seen from FIG. 2 that in the rest periods, that is to say thosetimes when no voltage is obtained between points 15 and 25, thecontrolled rectifiers of the first pair, rectifiers 12 and 22 in thepresent example, are maintained continuously in conduction. During thoseperiods at which power is to be supplied to the inverter output, one ofthese conducting rectifiers is extinguished and the rectifier of thesecond pair in series with it is fired. Thus, for example, in the firstvertical column of FIG. 2, rectifiers 12 and 22 are initiallyconducting. Rectifier 12 is then extinguished and simultaneouslyrectifier 11 is switched into conduction. There is thus obtained betweenpoints 15 and 25 a direct voltage with positive polarity and ofamplitude E.

When no voltage is obtained between points 15 and 25, the load is heldat the potential of terminal 2 of the supply through the controlledrectifier pair maintained continuously in conduction except during thoseperiods control circuits.

In FIGS. 3 and 4, corresponding respectively to low load and full loadon the inverter, waveforms a to j correspond respectively to the currentflowing through circuit elements 11, 13, 12, 14, 21, 23, 22, 24, thevoltage obtained between points and 25, and the current flowing throughthe load 3 after filtering and shaping, the load circuit being tuned tothe output frequency of the inverter.

It is seen from FIGS. 3 and 4 that one advantage of the invention liesin the fact that the diodes l3 and 23 do not carry any current, with thepossible exception of pulses during switching which are shown in dottedlines in the Figures. These two diodes may therefore have a smallercapacity than the diodes l4 and 24, thereby reducing the cost of theequipment.

Furthermore, when the load circuit is tuned to a frequency at leastequal to the output frequency of the inverter, the controlled rectifiers12 and 22 operate in natural extinction, that is, they cut off when thecurrent flowing between points 15 and passes through zero. Thus thecut-off or extinction energy of these two controlled rectifiers may beconsiderably reduced.

.A further advantage of the invention is that the measurement of thereactive current, passing to the load during the period when the sourceis not supplying energy to the load, is greatly facilitated. Thismeasurement may be accomplished by merely measuring the current throughthe protective diodes l4 and 24 which are connected to the pole of thesource to which the load is connected during the periods ofnon-energization. The current may be measured through a commonconnecting wire linking the diodes to the poles.

A still further advantage is that the battery used to supply the voltageE may be made smaller and two separate batteries may be used. This isaccomplished by eliminating the connection between diodes 11 and 21 andconnecting one source between terminals 1 and 2 and a second sourcebetween the anode of controlled rectifier 21 and the cathode ofcontrolled rectifier 22. This ispossible since terminal 2 is always thereference terminal when the load is not being energized. In prior artsystems, since the reference terminal would switch between terminals 1and 2, the use of two separate sources would not be possible.

Fig. 5 shows a first form of control circuitry forthe controlledrectifiers, arranged to provide a first train of pulses at twice theinverter output frequency and a second train of pulses delayed withrespect to the first train by an amount dependent on the alternatingoutput voltage of the inverter. In the Figure, blocks 10 and 20represent the two series-connected combinations of rectifiers on eachside of the load 3, comprising respectively controlled rectifiers 11 and12 with their diodes l3 and 14, and controlled rectifiers 21 and 22 withtheir diodes 23 and 24.

A generator circuit 31 provides two pulse trains with a relative delayof one half period of the output frequency of the inverter. The pulsetrains are applied to respective circuits 16 and 26, including, forexample, a monostable flip-flop forming a pulse which can be delayedwith respect to the input pulse by a variable amount. The pulse trainsfrom generator circuit 31 also are applied to the series-connectedcombinations l0 and 20, the pulse trains obtained from the left-handoutput of circuit 31 cutting off the controlled rectifier l2 and firingcontrolled rectifier 11, and the pulse trains obtained from theright-hand output controlling the combination 20 in an analogous manner.

These pulses constitute the first train at twice the inverter outputfrequency. The second train of pulses delayed with respect to the firsttrain'by an amount dependent on the alternating output voltage of theinverter are the pulse trains provided by the respective flip-flops l6and 26. These serve to reverse the action of the first train of pulses,to refire the extinguished rectifier of each combination and extinguishthe fired rectifier. The delay is controlled by an output of acomparator circuit 32, which is connected to sample the voltage acrossthe load 3, and arranged to compare this voltage with a referencepotential. An output voltage of the comparator depending on thedifference between the load voltage and the reference potential isapplied to each flip-flop l6 and 26.

FIG. 6 shows a second form of control circuitry. A generator circuit 33provides two triangular waveforms with a relative phase shift of onehalf period of the output frequency of the inverter. Each of thesetriangular waveform signals is clipped in a respective circuit 17 and27. The resulting pulse trains l9 and 29 are applied to respectivepulse-shaping circuits 18 and 28. These provide two control signalscorresponding to the leading edge and the end of the pulses 19 and 29.The

clipping level is controlled by an error signal provided the invention.

What I claim is:

1. An inverter comprising two pairs of controlled rectifiers connectedin a bridge circuit, an input across a first diagonal of said bridge forconnection to a direct current supply and an output across a seconddiagonal of said bridge for connection to an alternating current load,the controlled rectifiers of one pair having their anodes connected to apositive terminal of the input and the controlled rectifiers of theother pair having their cathodes connected to a negative terminal of theinput, means for controlling the firing time of said rectifiers suchthat the rectifiers of a first pair are maintained continuously inconduction during all periods when no power is to be supplied to theoutput and one of the conducting rectifiers is extinguished and therectifier of the second pair in series with it is fired during periodswhen power is supplied to the output, whereby the output voltage of theinverter is controlled by the firing times of the controlled rectifiers.

2. An inverter as claimed in claim 1, wherein the means for controllingthe firing time of said rectifiers provides a first train of pulses attwice the inverter output frequency and a second train of pulses delayedwith respect to the first train by an amount dependent on thealternating output voltage of the inverter, the pulses of firedrectifier.

3. An inverter as claimed in claim 1, wherein the means for controllingthe firing time of said rectifiers provides a first and second train ofpulses having periods equal to that of the alternating inverter outputvoltage, the leading edge of each pulse extinguishing one of thecontinuously conducting rectifiers of the first pair and firing therectifier of the second pair in series with it and the end of each pulsereversing that action refiring the extinguished rectifier andextinguishing the fired rectifier.

4. An inverter as claimed in claim 3, wherein the pulse trains areproduced by superposing a triangular waveform voltage with a directvoltage, wherein the amplitude of said direct voltage depends on thealternating output voltage of the inverter.

5. An inverter as claimed in claim 1, further comprising a diodeconnected in parallel with each of said controlled rectifiers.

6. Apparatus as claimed in claim 1, further comprising a comparatormeans connected to sample the inverter output voltage, compare saidvoltage with a reference potential and to provide an output signalindicative of the inverter output voltage.

1. An inverter comprising two pairs of controlled rectifiers connectedin a bridge circuit, an input across a first diagonal of said bridge forconnection to a direct current supply and an output across a seconddiagonal of said bridge for connection to an alternating current load,the controlled rectifiers of one pair having their anodes connected to apositive terminal of the input and the controlled rectifiers of theother pair having their cathodes connected to a negative terminal of theinput, means for controlling the firing time of said rectifiers suchthat the rectifiers of a first pair are maintained continuously inconduction during all periods when no power is to be supplied to theoutput and one of the conducting rectifiers is extinguished and therectifier of the second pair in series with it is fired during periodswhen power is supplied to the output, whereby the output voltage of theinverter is controlled by the firing times of the controlled rectifiers.2. An inverter as claimed in claim 1, wherein the means for controllingthe firing time of said rectifiers provides a first train of pulses attwice the inverter output frequency and a second train of pulses delayedwith respect to the first train by an amount dependent on thealternating output voltage of the inverter, the pulses of the firsttrain serving to extinguish one of the continuously conductingrectifiers of the first pair and to fire the rectifier of the secondpair in series with it, and the pulses of the second train serving toreverse this action to refire the extinguished rectifier and extinguishthe fired rectifier.
 3. An inverter as claimed in claim 1, wherein themeans for controlling the firing time of said rectifiers provides afirst and second train of pulses having periods equal to that of thealternating inverter output voltage, the leading edge of each pulsEextinguishing one of the continuously conducting rectifiers of the firstpair and firing the rectifier of the second pair in series with it andthe end of each pulse reversing that action refiring the extinguishedrectifier and extinguishing the fired rectifier.
 4. An inverter asclaimed in claim 3, wherein the pulse trains are produced by superposinga triangular waveform voltage with a direct voltage, wherein theamplitude of said direct voltage depends on the alternating outputvoltage of the inverter.
 5. An inverter as claimed in claim 1, furthercomprising a diode connected in parallel with each of said controlledrectifiers.
 6. Apparatus as claimed in claim 1, further comprising acomparator means connected to sample the inverter output voltage,compare said voltage with a reference potential and to provide an outputsignal indicative of the inverter output voltage.